For many years it has been known that certain metals, such as titanium and many of its alloys, exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with reduced tendency towards necking. This capability is exhibited by only a few metals and alloys and within limited temperature and strain rate range. An example of the superplastic forming process is disclosed in U.S. Pat. No. 3,340,101 to Fields, Jr., et al.
In our U.S. Pat. No. 3,920,175, there is disclosed a method for superplastic forming of metals with concurrent diffusion bonding. This patent is generally directed to superplastically deforming a metal blank against a metal workpiece which is bonded to the metal blank. However, this patent does not disclose fabrication of honeycomb structures where a face sheet having superplastic characteristics is deformed against a honeycomb core and another face sheet such that metallurgical bonding of the parts to one another results and without crushing or significant deformation of the core so that the desired honeycomb metallic structure is formed.
The prior art method of forming honeycomb structures normally involves separate forming of the face sheets, core, and edge members which has required costly preforming and very close tolerances, fitting up the parts, sealing of edge members, and brazing the face sheets to the core and edge member structure, with the accompanying contamination and flow problems of brazing. Thus, the prior art honeycomb structure is made up of numerous separate parts which must be individually formed and brazed together. Such parts have not proven overly reliable, have been costly to fabricate, and are not weight efficient.
The present invention is directed to a method for fabricating honeycomb metallic structures which combines superplastic forming and metallurgical bonding. More specifically, the present invention is directed to a method where a core is positioned between two metal blanks, with one of the metal blanks being superplastically formed against the core and other matel blank such that the parts are metallurgically bonded to one another to form a desired honeycomb structure.